Method for locating devices

ABSTRACT

The disclosure relates to a process for locating devices, the process comprising the following steps:
         a) providing a carrier substrate comprising:
           a device layer; and   alignment marks;   
           b) providing a donor substrate;   c) forming a weak zone in the donor substrate, the weak zone delimiting a useful layer;   d) assembling the donor substrate and the carrier substrate; and   e) fracturing the donor substrate in the weak zone so as to transfer the useful layer to the device layer;   wherein the alignment marks are placed in cavities formed in the device layer, the cavities having an aperture flush with the free surface of the device layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national phase entry under 35 U.S.C. §371 ofInternational Patent Application PCT/FR2014/051568, filed Jun. 24, 2014,designating the United States of America and published in English asInternational Patent Publication WO 2015/007971 A1 on Jan. 22, 2015,which claims the benefit under Article 8 of the Patent CooperationTreaty and under 35 U.S.C. §119(e) to French Patent Application SerialNo. 1301697, filed Jul. 15, 2013, the disclosure of each of which ishereby incorporated herein in its entirety by this reference.

TECHNICAL FIELD

This disclosure relates to a process for locating devices after transferof a useful layer to a carrier substrate.

BACKGROUND

A prior-art process for locating devices after transfer of a usefullayer 8 to a carrier substrate 1, as illustrated in FIG. 1A, comprisesthe following steps:

-   -   a0) Providing a carrier substrate 1 comprising:        -   A front side 2;        -   A back side 3 parallel to the front side 2; and        -   A device layer 4 placed on the front side 2, the device            layer 4 comprising alignment marks 5;    -   b0) Providing a donor substrate 6;    -   c0) Forming a weak zone 7 in the donor substrate 6, the weak        zone 7 delimiting a useful layer 8;    -   d0) Assembling the donor substrate 6 and the carrier substrate        1; and    -   e0) Fracturing the donor substrate 6 in the weak zone 7 so as to        transfer the useful layer 8 to the device layer 4.

The device layer 4 comprises devices such as transistors, (npn or pnp)junctions, interconnections and any other structure. The useful layer 8generally comprises an opaque semiconductor layer.

After the fracturing step e0), the useful layer 8 masks the devices ofthe device layer 4 and the alignment marks 5.

The main drawback of this process is that the alignment marks 5 are nolonger accessible or observable.

Therefore, such as shown in FIG. 1B, which illustrates a commonpractice, apertures 9 (or holes) must be produced in the useful layer 8in order to expose the alignment marks 5.

The process for forming the apertures 9 generally comprises aphotolithography step, followed by an etching step.

The photolithography step is intended to define the shape and positionof the apertures 9 in the useful layer 8. However, this step, carriedout with no reference point other than the edge of the carrier substrate1, has a precision of +/−100 μm. Therefore, the alignment marks 5 can belocalized only to within +/−100 μm. The apertures 9 must thus have aside length (i.e., a width) of about 250 μm. Such a side length consumestoo much space, and is unacceptable.

One aim of the disclosure is, therefore, to provide a simpler processfor locating devices after transfer of a useful layer 8, allowingsmaller apertures to be formed than with prior-art techniques.

BRIEF SUMMARY

This disclosure aims to remedy the aforementioned drawbacks, and relatesto a process for locating devices after transfer of a useful layer, theprocess comprising the following steps:

-   -   a) providing a carrier substrate comprising:        -   a device layer comprising a free surface; and        -   alignment marks;    -   b) providing a donor substrate;    -   c) forming a weak zone in the donor substrate, the weak zone        delimiting a useful layer;    -   d) assembling the donor substrate and the carrier substrate; and    -   e) fracturing the donor substrate in the weak zone so as to        transfer the useful layer to the device layer.

The alignment marks are placed in cavities formed in the device layer,the cavities having an aperture flush with the free surface of thedevice layer. The alignment marks are placed so as to make it possibleto locate the devices.

The useful layer may be formed from a set of sublayers. The useful layeris generally opaque and, therefore, masks the device layer after stepe).

The cavities have walls, and the volume delimited by the walls of acavity and its aperture makes up the volume of the cavity.

By “alignment marks placed in cavities,” is meant that the alignmentmarks are placed in the volume of the cavities.

The presence of through-holes in the useful layer has been observedafter the fracturing step e), and in positional correspondence with thecavities. The holes thus extend through the cavities into the usefullayer, so that the alignment marks are visible from the free surface ofthe useful layer.

Thus, the devices of the device layer may be located from the freesurface of the useful layer.

Furthermore, it is not necessary to execute fabrication steps specificto the formation of holes in the useful layer. The holes areautomatically formed in the positions of the cavities at the moment ofthe transfer of the useful layer.

According to one method of implementation, the assembly step d)comprises a direct bonding step executed in an environment at a pressurebelow 20 mbars.

Thus, it has been observed that the through-holes in the useful layerhave a shape that corresponds to the aperture of the cavities.Therefore, the through-holes in the useful layer do not encroach beyondthe aperture of the cavities.

According to one method of implementation, the cavities extend as far asinto the carrier substrate.

According to one method of implementation, the alignment marks areplaced at the bottom of the cavities.

According to one method of implementation, the device layer comprisesdevices regularly distributed over the entire extent of the devicelayer.

According to one method of implementation, an opaque layer is present onthe useful layer before the assembly step d).

According to one method of implementation, the opaque layer comprises atleast one material selected from the following group: tungsten,titanium, tungsten silicide, titanium silicide, nickel silicide, nickelsilicide and platinum.

According to one method of implementation, step c) of forming the weakzone is executed by implanting at least one of the species selected fromthe following group: hydrogen and helium.

According to one method of implementation, the fracturing step e)comprises a heat treatment executed at a temperature between 200° C. and500° C.

According to one method of implementation, the thickness of the usefullayer is smaller than 8000 Å and preferably smaller than 5000 Å.

According to one method of implementation, the useful layer comprisessublayers of different doping.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages will become apparent from the followingdescription of methods of implementation of a process for locatingdevices according to the disclosure, which are given by way ofnonlimiting example and with reference to the appended drawings, inwhich:

FIGS. 1A and 1B are schematic representations of a structure obtainedwith a process for locating devices according to prior-art techniques;

FIG. 2 is a schematic representation of a process for locating devicesaccording to one embodiment of the disclosure;

FIG. 3 is a schematic representation of the structure obtained with theprocess for locating devices according to a second embodiment of thedisclosure; and

FIG. 4 is a schematic representation of the process for locating devicesaccording to one embodiment of the disclosure.

DETAILED DESCRIPTION

For the various methods of implementation, the same references will beused for elements that are identical or that provide the same function,for the sake of simplicity of the description.

FIGS. 2 and 3 schematically illustrate a process for locating devices.

To facilitate the illustration, the respective thicknesses of thevarious layers are not shown to scale.

Step a) of the process for locating devices comprises providing acarrier substrate 10 comprising a front side 20 and a back side 30parallel to the front side 20.

The carrier substrate 10 may comprise a bulk substrate on which a devicelayer 40 is formed on the front side 20 of the carrier substrate 10.

The bulk substrate may consist of any material conventionally used inthe microelectronics, optics, optoelectronics and photovoltaicindustries.

In particular, the bulk substrate may comprise at least one materialselected from the following group: silicon, silicon carbide, silicongermanium, glass, a ceramic and a metal alloy.

The device layer 40 comprises devices, such as electronic devices (forexample transistors, junctions, etc.), interconnections and/or metalizedzones.

The devices are formed using techniques well known to those skilled inthe art.

The devices may be regularly distributed over the entire extent of thedevice layer 40.

Cavities 90 are formed in the device layer 40. The cavities 90 are openand comprise an aperture flush with the free surface of the device layer40.

The cavities 90 comprise walls. The walls of the cavity 90 and theaperture of the cavity 90 delimit the volume of the cavity 90.

Advantageously, the cavities 90 may extend as far as into the carriersubstrate 10.

Alignment marks 50 are placed in the volume of the cavities 90, and thealignment marks 50 are away from the aperture of the cavities 90.

The alignment marks 50 are placed so as to make it possible to preciselylocate the devices of the device layer 40.

Alignment marks 50 are conventionally used to align photolithographymasks.

The alignment marks 50 may take the form of crosses, chevrons orinterference patterns, or any other form liable to allow the preciselocation of the devices to be determined.

In this respect, those skilled in the art will find a technicaldescription of the alignment or photolithography masks in Fundamentalsof Microfabrication: The Science of Miniaturization, 2^(nd) ed., Marc J.Madou, Nanogen, Inc., San Diego, Calif.

Advantageously, the alignment marks 50 are placed at the bottom of thecavities 90.

Step b) of the process for locating devices comprises providing a donorsubstrate 60.

The donor substrate 60 may be made of any material conventionally usedin the microelectronics, optics, optoelectronics and photovoltaicindustries.

In particular, the donor substrate 60 may comprise at least one materialselected from the following group: silicon, silicon carbide and silicongermanium.

The donor substrate 60 may comprise a semiconductor material.

Step c) of the process for locating devices may comprise forming a weakzone 70 in the donor substrate 60.

The donor substrate 60 comprises a first surface. The weak zone 70 andthe first surface of the donor substrate 60 delimit a useful layer 80intended to be transferred to the device layer 40.

The weak zone 70 may be obtained by implanting atomic species. Theweakening implantation may be carried out with a single species (forexample, hydrogen or helium), but may also be carried out with aplurality of sequentially implanted species (for example, hydrogen andhelium).

Advantageously, the hydrogen is implanted with an energy of between 20and 70 keV, and a dose of between 4×10¹⁶ and 6×10¹⁶ atoms/cm².

The helium may be implanted with an energy of between 20 and 70 keV, anda dose of between 0.5×10¹⁶ and 3×10¹⁶ atoms/cm².

In some embodiments, the useful layer 80 may have a thickness smallerthan 8000 Å and preferably smaller than 5000 Å.

Step d) of the process for locating devices may comprise assembling thedonor substrate 60 and the carrier substrate 10.

In some embodiments, the assembly step d) may be executed by directbonding.

The assembly may be executed by bringing the useful layer 80 into directcontact with the device layer 40. The assembly is executed so as topreserve, at least in part, the volume of the cavities 90. Therefore,the presence of cavities 90 generates unbonded zones.

The assembly step d) may comprise a direct bonding step executed in anenvironment at a pressure below 20 mbar (2000 Pa).

Such as illustrated in FIG. 4, an intermediate layer 100 may be placedon the useful layer 80 before the assembly step d).

The intermediate layer 100 may be an opaque layer placed on the usefullayer 80.

The opaque layer may comprise at least one material selected from thefollowing group: tungsten, titanium, tungsten silicide, titaniumsilicide, nickel silicide, nickel silicide and platinum.

Step e) of the method for locating devices comprises fracturing thedonor substrate 60 in the weak zone 70 so as to transfer the usefullayer 80 to the device layer 40.

Thus, after step e), the useful layer 80 masks the devices.

If an opaque layer has been formed on the useful layer 80 prior to theassembly step d), the opaque layer is then located intermediate betweenthe useful layer 80 and the device layer 40 after the fracturing step.

Advantageously, the fracturing step e) may comprise a heat treatmentexecuted at a temperature between 200° C. and 500° C.

Particularly advantageously, after the fracturing step e), the presenceof through-holes has been observed in the useful layer 80, and theopaque layer in the case where the latter is present.

Moreover, the through-holes in the useful layer 80 are in positionalcorrespondence with the cavities 90, so that the through-holes extendthe cavities 90 into the useful layer 80.

Furthermore, the aperture of each cavity 90 is inscribed in the apertureof a hole of the useful layer 80.

In embodiments in which step d) comprises direct bonding executed in anenvironment at a pressure below 20 mbar (2000 Pa), each through-hole inthe useful layer 80 may have an aperture that corresponds to theaperture in the cavity 90 into which it extends, such as illustrated inFIG. 3.

The presence of these holes in the useful layer 80 is advantageouslycapitalized upon to expose the alignment marks 50 placed in the cavities90.

Thus, it is possible to locate the devices masked by the useful layer80.

1. A process for locating devices after transfer of a useful layer, theprocess comprising the following steps: a) providing a carrier substratecomprising: a device layer comprising a free surface; and alignmentmarks; b) providing a donor substrate; c) forming a weak zone in thedonor substrate, the weak zone delimiting a useful layer; d) assemblingthe donor substrate and the carrier substrate; and e) fracturing thedonor substrate in the weak zone so as to transfer the useful layer tothe device layer; wherein the alignment marks are placed in cavitiesformed in the device layer, the cavities having an aperture flush withthe free surface of the device layer.
 2. The process according to claim1, in which the assembly step d) comprises a direct bonding stepexecuted in an environment at a pressure below 20 mbars.
 3. The processaccording to claim 2, wherein the cavities extend into the carriersubstrate.
 4. The process according to claim 3, wherein the alignmentmarks are placed at the bottom of the cavities.
 5. The process accordingto claim 4, wherein the device layer comprises devices distributed overan entire extent of the device layer.
 6. The process according to claim4, wherein an opaque layer is present on the useful layer before theassembly step d).
 7. The process according to claim 6, wherein theopaque layer comprises at least one material selected from the followinggroup: tungsten, titanium, tungsten silicide, titanium silicide, nickelsilicide, nickel silicide and platinum.
 8. The process according toclaim 4, wherein the step c) of forming the weak zone is executed byimplanting at least one of the species selected from the followinggroup: hydrogen and helium.
 9. The process according to claim 4, whereinthe fracturing step e) comprises a heat treatment executed at atemperature between 200° C. and 500° C.
 10. The process according toclaim 1, wherein the thickness of the useful layer is smaller than 8000Å.
 11. The process according to claim 1, wherein the useful layercomprises sublayers of different doping.
 12. The process according toclaim 10, wherein the thickness of the userful layer is smaller than5000 Å.
 13. The process according to claim 1, wherein the cavitiesextend into the carrier substrate.
 14. The process according to claim 1,wherein the alignment marks are placed at the bottom of the cavities.15. The process according to claim 1, wherein the device layer comprisesdevices distributed over an entire extent of the device layer.
 16. Theprocess according to claim 1, wherein an opaque layer is present on theuseful layer before the assembly step d).
 17. The process according toclaim 1, wherein the step c) of forming the weak zone is executed byimplanting at least one of the species selected from the followinggroup: hydrogen and helium.
 18. The process according to claim 1,wherein the fracturing step e) comprises a heat treatment executed at atemperature between 200° C. and 500° C.